Such apparatuses are applied, for example, for determining the carbon content and/or the nitrogen content of wastewater. Some of the most important constituents to be determined in waste waters are the following:    TC (Total Carbon, total carbon content), the entire amount of carbon contained in the aqueous liquid;    TOC (Total Organic Carbon, total organically bound carbon), the entire amount of carbon contained in the form of organic compounds in the aqueous liquid;    TIC (Total Inorganic Carbon, total inorganically bound carbon), the entire amount of carbon contained in the form of inorganic compounds in the aqueous liquid;    TNb, (Total Nitrogen, total bound nitrogen), the entire amount of bound nitrogen contained in the aqueous liquid.
In the case of known methods, a liquid sample of small volume of, for example, a few 100 μl is fed to the high temperature reactor. In determining TOC, there occurs, in given cases, a pretreating for removing the inorganic compounds, e.g. by acidification, wherein carbon dioxide (CO2) is given off and removed by purging. In the high temperature reactor, the organic constituents are thermally decomposed to CO2, and the nitrogen containing constituents to nitrogen oxides NOx. The acronym NOx represents a mixture of nitrogen oxides of different degrees of oxidation, which has, however, NO as the main component, namely at about 98%. In a smaller amount, the mixture also contains NO2; however, conversion of the NO into NO2 under the given conditions takes a relatively long time. In the decomposition in the high temperature reactor, there arises a gas mixture, which contains, besides CO2 and NOx, gaseous H2O, as well as impurities, for example, in the form of sublimated salts or metal oxides. The gas mixture is transported with the assistance of a carrier gas (which, as a rule, also delivers the oxygen needed for reaction) through a cooler with water separator, a gas filter and an analytical unit. The amount of the arising CO2, or NOx, is ascertained, for example, by infrared or chemiluminescent measurement, and, from this value, the TOC or TNb is calculated.
For liquid samples, the reaction temperature is usually about in the range of 680° C. to 1000° C., wherein higher temperatures, in principle, favor the sample decomposition. Complete conversion of the carbon, or nitrogen, compounds contained in the sample is supported through the use of catalysts, for example, platinum, or oxygen transferees, e.g. ceroxide or nickel oxide. It is also possible to perform the decomposition of the sample without catalyst or oxygen transferer. In this case, the reaction is, as a rule, performed at temperatures of more than 1000° C.
Contained in the liquid samples and therewith also correspondingly in the gas mixture occurring in the high temperature reactor are frequently matrix components, such as: Salts, e.g. chloride, sulfates or phosphates, which, at the temperatures reigning in the reaction zone of the high temperature reactor, transfer by sublimation into the gas phase; metal oxides, which can be present as solid particles within the gas mixture; and acids or basic components. These matrix components can disturb or even corrupt the determining of the oxidizable constituents. For example, in cold regions of the apparatus, desublimated salts and metal oxide particles can separate from the gas mixture and lead to the clogging of narrow gas lines or filters. Moreover, corrosive gases, vapors or aerosols can arise from the salts or other components in the gas phase, and be distributed by the carrier gas into the regions of the apparatus, through which the carrier gas flows. In this way, sensitive parts of the apparatus, especially the detectors present for the analysis, can be damaged.
Measures are known from the state of the art for reducing these damaging effects of the matrix components, especially the salts.
Thus, DE 44 17 247 B4 discloses an analytical apparatus having a combustion furnace for liquid samples for determining the content of oxidizable constituents, in which an insert is arranged, which contains filler elements of quartz glass or ceramic material. The insert is arranged in a region of the combustion furnace, in which the operating temperature lies between 100° C. and 400° C. At these temperatures, the salts present in the gas mixture deposit on the filler elements and, therefore, do not get into the remaining regions of the analytical apparatus.
From time to time, the insert must, however, be replaced or deinstalled and cleaned, since its ability to catch the precipitated salts is limited. This requires a complex sealing technology, especially since the sealing surfaces at the position of the insert in the combustion furnace must be designed for high temperatures. During the replacement of the insert, the combustion furnace must be cooled down, which leads to a down time of the analytical apparatus. When taking out the insert to be replaced or regenerated, there is a danger that salt particles can fall out of the insert and foul the apparatus. Especially damaging is the situation, when the salt particles reach the sealing surfaces, since then a reliable sealing in the case of renewed start-up is no longer assured.
In U.S. Pat. No. 4,078,894, an apparatus for waste water analysis with a high temperature reactor is shown, in which a liquid sample is converted to a gas mixture by heating in the presence of a carrier gas acting as oxidizing agent. The gas discharge of the reactor opens into a chamber, which is filled in a lower region with water, and in which the gas mixture is so greatly cooled off, that therein contained salts fall out and are dissolved in the water. By pumping the water from the chamber, the salts are removed from the system. Interaction of the condensate with the high velocity gas stream flowing past can, however, lead to the formation of damaging aerosols in the case of such an apparatus.
In JP 2007-054791 A, an apparatus to treatment of wastewater by means of burning is shown, to which is connectable an analytical device for determining various constituents in the process off-gas. In order not to corrupt the measuring, it is necessary that, before introduction of the process off-gas into the analytical device, salt containing fog, or aerosols be removed from the process off-gas. This occurs by means of a gas cleaning unit (scrubber) with, connected in series, a fine filter, which are arranged together in a housing. The housing is heated to a temperature above the condensation temperature of the water content in the process off-gas and below 150° C., in order to avoid condensing of water from the process off-gas. The removal of aerosols from an apparatus is, however, associated with relatively high apparatus complexity.